Improvement in thermal efficiency of a steam turbine used in a thermal power station and the like has become an important task leading to efficient use of energy resources and a reduction in carbon dioxide (CO2) emission. Effectively converting given energy to mechanical work makes it possible to achieve the improvement in thermal efficiency of a steam turbine. To achieve this, reducing various internal losses is required.
The internal losses of the steam turbine include a profile loss resulting from a blade shape, turbine cascade losses based on a secondary flow loss of steam, a leakage loss of steam, a moisture loss of steam, and so on, passage part losses in passages other than a cascade represented by a steam valve and a crossover pipe, turbine exhaust losses resulting from a turbine exhaust chamber, and so on.
Among these losses, the turbine exhaust loss is a large loss occupying 10% to 20% of all of the internal losses. The turbine exhaust loss is a loss generated from an outlet of a final stage of turbine stages to an inlet of a condenser. The turbine exhaust losses are further classified into a leaving loss, a hood loss, an annular area restriction loss, a turn-up loss, and so on. Among them, the hood loss is a pressure loss from an exhaust chamber to a condenser. The hood loss depends on a type, a shape, and a size of the exhaust chamber including a diffuser.
Generally, the pressure loss increases in proportion to the square of a flow velocity of the steam. Therefore, it is effective to reduce the flow velocity of the steam by increasing the size of the exhaust chamber in an allowable range. However, the increase in the size of the exhaust chamber is restricted by manufacturing cost, arrangement space of a building, and so on. When the size of the exhaust chamber is increased to reduce the hood loss, there are the above-stated restrictions. Besides, the hood loss depends on an axial velocity being a velocity in a turbine rotor axial direction, in other words, a volume flow rate passing through the exhaust chamber.
The hood loss depends on a design of the exhaust chamber including the diffuser. An exhaust chamber of a low-pressure turbine occupies a large capacity in a whole of the steam turbine. Accordingly, the increase in the size of the exhaust chamber to reduce the hood loss largely affects on a whole size and the manufacturing cost of the steam turbine. Therefore, it is important to enable a shape whose pressure loss is small within the limited size of the exhaust chamber.
In a double-flow exhaust type (double flow type) low-pressure turbine including a conventional exhaust chamber in a downward exhaust type, steam passing through a rotor blade of a final turbine stage is led to an annular diffuser made up of a steam guide and a bearing cone. The steam led to the diffuser flows out radially toward outside in a radial direction. A flow of the steam flowing out radially is turned by a casing and so on, and the steam is led to the condenser provided at downward of the steam turbine.
In the low-pressure turbine as stated above, it is important to decelerate the flow at the annular diffuser and to enough recover a static pressure to reduce the pressure loss (static pressure loss) in the exhaust chamber. However, in the low-pressure turbine as stated above, for example, when an inclination angle of an inner surface at an inlet of the steam guide relative to the turbine rotor axial direction is large, the steam separates at a position near an inlet in the diffuser. The separation as stated above remarkably occurs when the flow of the steam cannot be turned moderately in the diffuser, specifically, when a distance of the bearing cone in the turbine rotor axial direction is short.
Conventionally, an attempt to make a shape of a tip part (shroud) of the rotor blade at the final turbine stage into a shape steeply expanding toward outside in the radial direction to thereby suppress the separation of the flow at the steam guide has been done.
However, the suppression of the separation of the flow at the steam guide in the conventional steam turbine is not sufficient. Accordingly, a technology in which the pressure loss in the exhaust chamber is certainly reduced in the steam turbine has been required.